Andrey Shirokov, left, of Moscow State University in Russia, who has been a visiting scientist at Iowa State, and James Vary of Iowa State are part of an international team of nuclear physicists who theorized, predicted and announced a four-neutron structure in 2014 and 2016. Archive photo by Christopher Gannon for the College of Liberal Arts and Sciences.

Tetraneutron – An Exotic State of Matter Discovered

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. Only neutron stars represent nearly pure neutron systems, where neutrons are squeezed together by gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades, with particular emphasis on the four-neutron system called the tetraneutron, which has so far given only a few hints of its existence, leaving the tetraneutron an elusive nuclear core. system for six decades.

A recently announced experimental discovery of a tetraneutron by an international group led by scientists from the German Technical University of Darmstadt opens doors for further research and could lead to a better understanding of how the universe is put together. This new and exotic state of matter could also have useful properties in existing or emerging technologies.

The first announcement of the tetraneutron was made by theoretical physicist James Vary in a presentation in the summer of 2014, followed by a research paper in the fall of 2016. He is waiting to confirm the reality by nuclear physics experiments .

Now his wait is finally over when four neutrons are briefly bound together in a temporary quantum state.

What are neutrons?

Neutrons are chargeless subatomic particles that combine with positively charged protons to form the nucleus of an atom. Individual neutrons are not stable and after a few minutes turn into protons.

Why tetraneutrons?

The system made up of two neutrons, the dineutron, is known to be bound only by about 100 keV. Whether multineutron systems can exist as weakly bound states or very short-lived unbound resonant states is a long-standing question. The next simplest system of three neutrons is less likely to exist due to the odd number of nucleons and therefore weaker binding; however, a recent calculation has suggested its existence. Following these considerations, the four-neutron system, the tetraneutron, is a suitable candidate to answer this question.

On the way to the tetraneutron.

Using supercomputing power at Lawrence Berkeley National Laboratory in California, theorists calculated that four neutrons could form a resonant state with a lifetime of just 3×10^(-22) seconds, less than a billionth of a billionth of a second. It’s hard to believe, but it’s long enough for physicists to study.

Study details

Theorists’ calculations indicate that the tetraneutron should have an energy of about 0.8 million electron volts (a common unit of measurement in high-energy and nuclear systems). physics – visible light has energies of about 2 to 3 electron-volts.) Calculations also indicated that the width of the energy peak plotted showing a tetraneutron would be about 1.4 million electron-volts. Theorists published later studies indicating that the energy would probably be between 0.7 and 1.0 million electron-volts while the width would be between 1.1 and 1.7 million electron-volts. This sensitivity arose from the adoption of different candidates available for the interaction between neutrons.

A recently published paper in the journal Nature reports that experiments at the Radioactive Isotope Beam Factory at the RIKEN research institute in Wako, Japan, revealed that the energy and width of tetraneutrons were about 2.4, respectively. and 1.8 million electron volts. These are both more important than the theoretical results, but Vary said uncertainties in the current theoretical and experimental results could cover these differences.

Significance of the study

“A tetraneutron has such a short lifespan that it’s a big enough shock to the world of nuclear physics that its properties can be measured before it shatters,” Vary said. “It’s a very exotic system.”

It is, in fact, “a whole new state of matter,” he said. “It’s short-lived, but it points to possibilities. What happens if you put two or three together? Could you get more stability?

Experiments to find a tetraneutron began in 2002 when the structure was proposed in some reactions involving one of the elements, a metal called beryllium. A RIKEN team found hints of a tetraneutron in experimental results published in 2016.

“The tetraneutron will join the neutron as the second chargeless element in the nuclear diagram,” Vary wrote in a project summary. This “provides a valuable new platform for theories of strong neutron interactions”.

“Can we create a small neutron star on Earth? Vary titled a summary of the tetraneutron project. A neutron star is what remains when a massive star runs out of fuel and collapses into a super-dense neutron structure. The tetraneutron is also a neutron structure, an argument of Vary is a “short-lived and very light neutron star”.

“I had pretty much given up on the experiments,” Vary said. “I hadn’t heard anything about it during the pandemic. It was a big shock. Oh my God, here we are, we may actually have something new.

“We have presented the experimental observation of a resonance-like structure consistent with a near-threshold tetraneutron state after 60 years of experimental attempts to clarify the existence of this state.” The study ends.

Journal reference

  1. M. Duer, T. Aumann, R. Gernhäuser, V. Panin, S. Paschalis, DM Rossi, NL Achouri, D. Ahn, H. Baba, CA Bertulani, M. Böhmer, K. Boretzky, C. Caesar, N Chiga, A. Corsi, D. Cortina-Gil, CA Douma, F. Dufter, Z. Elekes, J. Feng, B. Fernández-Domínguez, U. Forsberg, N. Fukuda, I. Gasparic, Z. Ge, JM Gheller, J. Gibelin, A. Gillibert, KI Hahn, Z. Halász, MN Harakeh, A. Hirayama, M. Holl, N. Inabe, T. Isobe, J. Kahlbow, N. Kalantar-Nayestanaki, D. Kim, S. Kim, T. Kobayashi, Y. Kondo, D. Körper, P. Koseoglou, Y. Kubota, I. Kuti, PJ Li, C. Lehr, S. Lindberg, Y. Liu, FM Marqués, S. Masuoka, M. Matsumoto, J. Mayer, K. Miki, B. Monteagudo, T. Nakamura, T. Nilsson, A. Obertelli, NA Orr, H. Otsu, SY Park, M. Parlog, PM Potlog, S. Reichert, A Revel, AT Saito, M. Sasano, H. Scheit, F. Schindler, S. Shimoura, H. Simon, L. Stuhl, H. Suzuki, D. Symochko, H. Takeda, J. Tanaka, Y. Togano, T. Tomai, HT Törnqvist, J. Tscheuschner, T. Uesaka, V. Wagner, H. Yamada, B. Yang, L. Yang, ZH Yang, M. Yasuda, K. Yo ne da, L. Zanetti, J. Zenihiro & MV Joukov. Observation of a system with four correlated free neutrons. Nature 606, 678–682 (2022). DO I: 10.1038/s41586-022-04827-6

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